We've used Driscoll & Crowley a couple of times and found them to be really professional. I don't know how they compare on price - we had such a bad experience with some other plumbers we haven't really looked around since finding these people. I mean, there are only so many times you need a plumber! They seem to have a website here:

if your heating water is dirty and causing problems with your boiler it may be best to have a "Powerflush" on you central heating system.
Basically a water transfusion, removing the old water and any magnetite(sludge) with in the system. Also it may be worth installing an Fernox Boiler buddy or magna clean within your system to keep it free of sludge and circulating debris such as rust and ferrous particles.
An power flush would normally take a day Max. to perform and cost any thing between £250- £400.

For more information of how a boiler buddy works and the effects of a unprotected heating system can cause log onto fernox.co.uk